The ever increasing use of fossil fuels has led to the development of drilling technologies that were unimaginable in the recent past. For instance, the ability to determine the geological strata and the probability of drilling a producing well can be determined from sensing devices placed near the bit head of a well drill. In certain cases the delicate nature of the active element of a sensing device can place requirements on vibration-dampening mounting systems that state of the art technology is unable to meet.
In one example, a gamma radiation detecting sensor is mounted close to the bit head of a well drill. The sensitivity of nuclear logging equipment is directly related to the volume and therefore the diameter of the active sensing element. In the case of a gamma radiation detector, the active sensing element is typically a thallium-doped sodium iodide crystal that is sensitive to mechanical vibration and shock. If the active element receives too much vibration then false readings and a general degradation of the mechanically delicate active element can occur. In extreme cases, if the active element receives too great a shock then mechanical failure of the crystal sensing element can occur.
Obtaining maximum sensitivity of the active element requires that the portion of the sensing element containing the active element be assembled from relatively thin-walled components that do not permit the implementation of typical sensor suspension methods. Current methods of vibration dampening and suspension of logging sensors within cylindrical pressure housings generally rely on a series of large cross-section O-rings installed around the sensor housing along and perpendicular to the axial length of the sensor assembly. In another current method of vibration dampening and logging sensor suspension, a series of metallic leaf springs, extending along the axis of the sensor assembly, are installed between the outer surface of the sensor housing and the inner surface of the pressure housing.
The O-ring suspension method divides the sensor into sections that can be tuned to a high enough resonant frequency to be unaffected by the vibrations of typical operating conditions. The resonant frequency tuning requirement is at odds with the requirement to maximize the sensing element volume and therefore produces active element tube lengths longer than desirable between O-ring supports. In a typical installation, the sensing crystal is positioned at the center of an O-ring to O-ring gap and receives the maximum displacement from the induced vibrations. Consequently, as described above, sensor behavior ranging from false counts to sensing crystal failure can occur. Further, O-ring placement around the outer surface of the sensor assembly can interfere with the passage of the electrical conductors along the axial surface of the sensor assembly.
In the leaf spring suspension method, the leaf springs are fabricated from formed sheetmetal sections. Based on the mechanical properties of the sheetmetal, the stiffness required to produce a sufficiently high resonant frequency consequently produces a greater than desirable insertion/extraction force on the sensor assembly as it is inserted into or extracted from the pressure housing. The mechanical stresses therefore can result in deformation of the sensor assembly or significant shock to the sensor active element. In either case, premature failure of the active sensor element can occur. Further, the assembler should take care when inserting the sensor assembly into the pressure housing to prevent the leaf spring suspension system from damaging the electrical conductors running along the axial surface of the sensor assembly.
Under the above described well sensor operating conditions, a system and associated methods are desired allowing the damping of vibration while permitting the largest possible diameter crystal sensing element and its associated optically matched photomultiplier tube. The system should allow a longer useful life of the active sensing element and reduce the amount of error generated because of excessive vibrations and false counts. The system should provide uninterrupted and uncompromised passage of electrical conductors and permit visible inspection of the electrical harness. The ability for end user assembly and disassembly for sensor element servicing is also desired.